A method for cleaning viscose production off-gases and catalysts for use in the method

ABSTRACT

A method for cleaning an off-gas from viscose production, essentially containing H 2 S and CS 2 , comprises passing the gas through a catalytic reactor containing a direct oxidation type catalyst, such as V 2 O 3  on silica, to convert H 2 S in the gas to elemental sulfur, SO 2  or mixtures thereof, either via the oxygen present in the gas or via oxygen added to the gas stream. Elemental sulfur and SO 2  are removed from the effluent gas from the catalytic reactor, and the unconverted CS 2  is recycled to the viscose production process.

The present invention relates to a method for cleaning off-gases from viscose production and catalysts for use in the method.

Viscose rayon is a fiber of regenerated cellulose; it is structurally similar to cotton but may be produced from a variety of plants such as soy, bamboo, and sugar cane. Cellulose is a linear polymer of β-D-glucose units with the empirical formula (C₆H₁₀O₅)_(n). To prepare viscose, dissolving pulp is treated with aqueous sodium hydroxide (typically 16-19% w/w) to form “alkali cellulose”, which has the approximate formula [C₆H₉O₄—ONa]_(n). The alkali cellulose is then treated with carbon disulfide to form sodium cellulose xanthate:

[C₆H₉O₄—ONa]_(n) −nCS₂→[C₆H₉O₄—OCS₂Na]_(n)

The higher the ratio of cellulose to combined sulfur, the lower the solubility of the cellulose xanthate. The xanthate is dissolved in aqueous sodium hydroxide (typically 2-5% w/w) and allowed to depolymerize to a desired extent, indicated by the viscosity of the solution. The rate of depolymerization (ripening or maturing) depends on the temperature and is affected by the presence of various inorganic and organic additives, such as metal oxides and hydroxides. Air also affects the ripening process since oxygen causes depolymerization.

Rayon fibers are produced from the ripened solutions by treatment with a mineral acid, such as sulfuric acid. In this step, the xanthate groups are hydrolyzed to regenerate cellulose and release dithiocarbonic acid that later decomposes to carbon disulfide and water:

[C₆H₉O₄—OCS₂Na]_(2n) +nH₂SO₄→[C₆H₉O₄—OH]_(2n)+2nCS₂ +nNa₂SO₄

H₂COS₂→H₂O+CS₂

Aside from regenerated cellulose, acidification gives hydrogen sulfide, sulfur, and carbon disulfide. The thread made from the regenerated cellulose is washed to remove residual acid. The sulfur is then removed by the addition of sodium sulfide solution and impurities are oxidized by bleaching with sodium hypochlorite solution.

So in the viscose industry, there are several off-gases containing sulfur components. Streams that contain high levels of H₂S are often recovered and converted to sulfuric acid which, as mentioned, is a raw material for the production of the viscose fibers. There are also gases which originate from fume collection, said gases containing less than 2% H₂S and associated sulfur compounds. The level of H₂S is often in the range from 200 to 2000 ppm and the CS₂ level around 200 to 1000 ppm. This more lean off-gas is treated in the majority of the viscose plants, which are in operation today. In the most common process, the gas is passed to a caustic scrubber (operating with NaOH), where H₂S is removed from the gas phase, forming NaHS and Na₂S which follow the liquid discharge.

Downstream from the scrubber, a regenerative activated carbon filter is applied, where the CS₂ is absorbed and concentrated in the filter bed. Then it is desorbed using a mild temperature swing process and returned, optionally after being upconcentrated, as a valuable raw material to the viscose plant. If H₂S is present in the inlet gas to the activated carbon step, it will occupy the capacity of the activated carbon and disrupt the CS₂ recycle process which is important for the overall plant economics. This is the major reason for using a NaOH based caustic scrubber upstream from the activated carbon bed.

The H₂S scrubber will use a significant excess of caustic material to catch H₂S since a high pH is needed to capture

H₂S in such a scrubber. In the water phase, a mixture of NaHS and Na₂S will be present, which is cumbersome and costly to treat in a wastewater treatment plant and also unstable with respect to release of H₂S if temperature or pH is not very carefully controlled.

US 2005/0147554 A1 describes the use of a wet oxidation process to control the concentration of reduced sulfur compounds from a viscose process. The wet oxidation process oxidizes the reduced sulfur compounds to convert them to an alkali sulfate and/or thiosulfate species. The alkali sulfate stream is recycled to the viscose process. The oxidation is performed under elevated temperature and pressure conditions, such that the off-gas has an oxygen concentration between about 2% and about 18%, and the alkali sulfate stream has a pH of about 2 to 10.

In U.S. Pat. No. 6,416,725 B1, sulfurous gas streams comprising H₂S and CS₂, such as produced as a by-product of the process of rayon-forming, are processed to recover the components in a usable form. First the gas stream is contacted with aqueous NaOH to dissolve out H₂S and some of the CS₂. The dissolved CS₂ is driven off from the solution and condensed out as a liquid concentrate. Any CS₂ remaining in the gas stream is recovered, such as by condensation. The aqueous sodium sulfide solution, which remains from removal of CS₂, is concentrated and the pH is adjusted, as necessary, to a value at which the sodium sulfide is predominantly in the form of sodium bisulfide. The concentrated sodium bisulfide solution is capable of being re-used in the rayon-forming process along with the liquid concentrate of CS₂.

Various embodiments of waste gas recovery and adsorption systems for waste gases from viscose fiber production are described in Chinese utility model and patent applications CN 204107303 U, CN 204134460 U, CN 201115810 Y and CN 101219319 A, generally employing alkali cleaning/adsorption and condensation methods. While relevant, however, these CN applications do not anticipate the present invention.

The idea of the present invention is to use a direct oxidation type catalyst upstream from the activated carbon filter and thus convert H₂S to elemental sulfur, SO₂ or mixtures thereof. Elemental sulfur can be removed in a condenser, and SO₂ is easily removed in a caustic scrubber using standard caustic agents, such as NaOH or CaCO₃, in a mild excess compared to the stoichiometric demand, thereby creating sulfates that can easily be handled in a liquid discharge system. Direct oxidation type catalysts comprising a catalytically active material, such as V₂O₅, on a support, such as silica, are known in the art and described in e.g. WO 2015/082351, U.S. Pat. No. 4,277,458 and US 2012/0014854.

More specifically, the present invention concerns a method for cleaning off-gases from viscose production, said off-gases essentially containing air and sulfur components, mainly H₂S and CS₂, comprising the steps of

-   -   passing the off-gas through a catalytic reactor containing a         direct oxidation type catalyst to convert H₂S in the off-gas to         elemental sulfur, SO₂ or mixtures thereof, either via the oxygen         present in the gas or via oxygen added to the gas stream,     -   removing elemental sulfur and SO₂ from the effluent gas from the         catalytic reactor, and     -   recycling the unconverted CS₂ to the viscose production process.

The off-gas from viscose production generally contains H₂S in a concentration around 200 to 2000 ppm and CS₂ in a concentration around 200 to 1000 ppm.

The catalytic reactor can be operated so as to convert H₂S to elemental sulfur, SO₂ or mixtures thereof and simultaneously leave CS₂ substantially unconverted.

Preferably the elemental sulfur is removed in a condenser, and the SO₂ is removed in a caustic scrubber. The CS₂ is absorbed and concentrated in a regenerative activated carbon filter downstream from the caustic scrubber.

Suitable catalysts for use in the method include catalysts comprising oxides of Fe, Cr, Zn, Mn, V, Co, Ti, Bi, Sb, Cu or Mg or mixtures thereof supported on silica, alumina, titania, ceria, silicium carbide or activated carbon or mixtures thereof and optionally promoted by an alkali metal, preferably Na₂O. An especially preferred catalyst comprises V₂O₃ supported on silica.

Operation temperatures are in the range from 100 to 300° C., preferably from 160 to 260° C. 

1. A method for cleaning off-gases from viscose production, said off-gases essentially containing air and sulfur components, mainly H₂S and CS₂, comprising the steps of passing the off-gas through a catalytic reactor containing a direct oxidation type catalyst to convert H₂S in the off-gas to elemental sulfur, SO₂ or mixtures thereof, either via the oxygen present in the gas or via oxygen added to the gas stream, removing elemental sulfur and SO₂ from the effluent gas from the catalytic reactor, and recycling the unconverted CS₂ to the viscose production process.
 2. The method according to claim 1, wherein the elemental sulfur is removed in a condenser and the SO₂ is removed in a caustic scrubber.
 3. The method according to claim 1, wherein the CS₂ is absorbed and concentrated in a regenerative activated carbon filter downstream from the caustic scrubber.
 4. The method according to claim 3, wherein the absorbed CS₂ is desorbed and returned, optionally after being upconcentrated, as a raw material to the viscose production plant.
 5. The method according to claim 1, wherein the off-gas from viscose production contains H₂S in a concentration around 200 to 2000 ppm and CS₂ in a concentration around 200 to 1000 ppm.
 6. The method according to claim 1, wherein the operation temperature is in the range from 100 to 300° C., preferably in the range from 160 to 260° C.
 7. A catalyst for use in the method according to claim 1, which is a direct oxidation type catalyst comprising oxides of Fe, Cr, Zn, Mn, V, Co, Ti, Bi, Sb, Cu or Mg or mixtures thereof supported on silica, alumina, titania, ceria, silicium carbide or activated carbon or mixtures thereof, optionally promoted by an alkali metal.
 8. The catalyst according to claim 7, which comprises V₂O₅ supported on silica. 